1. Field of the Invention
This invention relates to the field of metallurgy, and particularly to the field of processing precipitation hardenable aluminum alloys.
2. Description of the Prior Art
A fine grain size tends to improve the mechanical properties of most structural materials. Additionally, formability can be improved by elimination of "orange peel" structure, and superplasticity realized in many alloys by providing a fine grain structure. For alloys which are susceptable to stress corrosion cracking such as many precipitation hardening aluminum alloys, a fine grain structure generally decreases the susceptibility to stress corrosion. However, grain refinement is difficult to achieve in aluminum alloys, and most attempts to obtain a fine grain size by conventional mechanical working and recrystallization by heating have only resulted in the material recrystallizing to the original coarse grain size with large "pancake" shaped grains.
A method for obtaining grain refinement for 7075 aluminum alloy is described in U.S. Pat. No. 3,847,681 to Waldman, Sulinski, and Marcus and reported in a paper by the same inventors entitled "The Effect of Ingot Processing Treatment on the Grain Size and Properties of Al Alloy 7075," Metallurgical Transactions, vol. 5, March 1974, pp. 573-584. The Waldman treatment requires a long-time high-temperature homogenization to precipitate chromium followed by slow cooling to precipitate Zn, Mg, and Cu. The 7075 aluminum alloy is then mechanically worked and recrystallized by heating to refine the grain size. This prior art method is limited to alloys containing specific elements such as chromium and does not create as fine a grain size as does the method of the present invention.
U.S. Pat. No. 4,092,181 to Paton and Hamilton (two of the three present inventors) describes a method of imparting a fine grain to precipitation hardening aluminum alloys. The present invention is an improvement of the earlier patented method in that it describes additional conditions for minimizing the grain size.